The present invention relates to a display process using a thermo-optical effect with a memory in a thin disc-like liquid crystal layer. It also relates to disc-like liquid crystals having such an effect.
It is known that the thermo-optical effect in a liquid crystal can be used for display purposes. Cholesteric and smectic liquid crystals are used in the prior art. The recently discovered disc-like liquid crystals would not appear likely to have a thermo-optical effect which can be used for display purposes.
Thus, the following conditions have to be fulfilled for such a use. The liquid crystal, placed in a thin layer between two transparent plates, is raised to a temperature which is a few degrees centigrade below the temperature at which the mesomorphic phase used (smectic or cholesteric) has a transition to another mesomorphic phase or a isotropic liquid phase. It is ensured that the layer is uniformly oriented either as a result of a prior treatment of the face of the supporting plates in contact with the layer or by applying an electrical field. It is therefore perfectly and uniformly transparent.
An inscription light beam, generally chosen in the near infrared range, is moduled in intensity by a signal transmitting the image to be inscribed and scans the cell point-by-point. When the energy applied at a point by the beam has proved adequate to locally heat the liquid crystal to beyond the transition point and providing that the cooling conditions favour a rapid return to the initial temperature, a microscopically ordered texture then forms, e.g. a focal conical structure in the case of smectic crystals, but is macroscopically disordered. In the case of certain cholesteric and certain smectic crystals, it is sufficiently diffuse to give a good contrast compared with the points where the transparency state has been maintained. This state is precisely that of the points where the light energy of the modulated beam has been inadequate to bring about the transition. Moreover, there is a "memory" effect and the information can be retained by the diffuse state and this can extend to beyond about 100 hours.
By projecting the image of the cell, e.g. by means of an optical device onto a screen, the non-diffuse points of the cell appear as bright points and the diffuse points as dark points.
Variants of the display method described hereinbefore exist. However, in all cases, the possibility of a display with a memory effect is only possible if the reverse transition obtained by rapid cooling creates a macroscopically disordered structure, which is also stable, as in the case of the focal conical structure of smectic crystals.
Another possibility for the advantageous use of liquid crystals for display purposes is based on the possibility of voluntarily eliminating the disorder created by the rapid cooling by applying for the duration of this rapid cooling period, a transverse alternating electric field with a frequency of about 1 kHz. Under the orienting influence of the field, the points which have undergone the transition return to the macroscopically ordered mesomorphic phase, i.e. to the transparent state. The use of a variable field then makes it possible to eliminate the modulation of the beam during the inscription phase.
Certain disc-like liquid crystals and in particular hexasubstituted triphenylene derivatives have the following mesophases:
a thread disc-like mesophase D.sub.F PA1 a mosaic texture mesophase D.sub.B PA1 in exceptional cases, other mesophases, such as the inclined disc-like phase.
The mesophase D.sub.F has an order in which only the parallelism of the small discs constituted by the liquid crystal molecules is strictly respected, except for the thermal agitation. Unlike in the case of the focal conical structure of smectic crystals, there is no law governing the distance between two groups of molecules located in two parallel planes. Such a structure is transparent and has no diffuse effect.
The mesophase D.sub.B corresponds to stacks of molecules in columns. In these columns, the molecules are parallel to one another and in particular to the axis of each column.
With regards to the electric field effect in phase D.sub.F, it is known that the molecular discs can be oriented due to the existence of a dielectric anisotropy which, in the case of hexasubstituted triphenylene derivatives is usually positive. Conversely, the mesophase D.sub.B is insensitive to the electric field.